1. Field of the Invention
This invention relates to spray nozzles in general and, more specifically, to an improved spray nozzle assembly which produces an even fan-shaped spray pattern.
2. Background of the Related Art
The desire to accurately control the particle size of fluid sprayed from a spray nozzle has been a challenge for nozzle manufactures for many years. The accurate control of particle size often improves the quality of the overall process employing the nozzle. For example, in the manufacture of continuously cast steel slabs, ingots, billets, or the like, spray nozzles are used to cool a casting as it passes from a mold. The more accurately coolant spray is applied to the surface of the steel, the less likely there will be uneven cooling of the casting. Uneven cooling can create internal stresses in the cast material which could result in stress fractures and, consequently, lost product.
Optimal cooling of a casting can be obtained if spray nozzles apply an even blanket of atomized cooling fluid to the surface of the casting such that the atomized particles readily and completely evaporate upon contact. The spray nozzles must be sufficiently adjustable so that the variables encountered in the casting process are accommodated. For example, the surface speed, attitude, and temperature of the casting are factors that must be considered in applying cooling fluid.
Purely hydraulic spray nozzle systems, wherein fluid is forced by high pressure through small orifices in a nozzle head, were initially used in the prior art to cool casting products. However, such systems did not sufficiently atomize the fluid. This led to excessive amounts of fluid on the surface of the casting surface which in turn caused imperfections in the casting and unusable product. Air-assisted nozzles were later developed and have substantially replaced hydraulic spray systems. Air assisted nozzles permit the distribution of relatively fine fluid spray, thereby consuming significantly less fluid and providing greater cooling per unit volume of cooling fluid than the earlier hydraulic nozzle systems.
An example of a basic air assisted spray nozzle is disclosed in German Patent No. 2,816,441. This apparatus includes an air pipe having a closed top end and a nozzle tip at its bottom end which defines an air mist spraying orifice. An air supply tube penetrates the outer wall of the air pipe adjacent the closed top end. A fluid pipe penetrates the closed top end of the air pipe and extends coaxially for a distance into the air pipe. Fluid is fed into the fluid pipe through the upper end thereof. Air is simultaneously fed into the tubular volume between the inner wall of the air pipe and the outer wall of the fluid pipe. The air and fluid mix in the lower portion of the air pipe. A disadvantage of this prior art nozzle is that the air and fluid do not efficiently mix together since each exits from a respective pipe in essentially parallel co-axial streams. Therefore, relatively long fluid and air pipes are required to effectively mix and atomize the fluid. Consequentially, the device is difficult to adapt to particular applications and ultimately results in a cumbersome and relatively expensive cooling system.
An improvement over this early German device is described in U.S. Pat. No. 4,349,156. This apparatus includes an elongated expansion chamber containing an impingement plate positioned parallel to the longitudinal axis of the chamber. Fluid flow is introduced at high velocity into the chamber at an angle perpendicular to the plate. The fluid strikes the impingement plate and breaks up into finely atomized particles. A high velocity air stream is directed into the chamber along the longitudinal axis thereof and strikes the fluid particles causing them to become further atomized. The atomized particles of fluid are carried along the length of the chamber by the high velocity air stream and exit the chamber through an orifice formed at its end. This apparatus has also been found to be ineffective because of the large amount of air that must be used to achieve a droplet size needed for the efficient and effective cooling required in continuous casting.
A further improvement in the development of spray nozzles is found in U.S. Pat. No. 4,511,087 ("the '087 patent"). This spray nozzle includes a nozzle tip at one end and a casing connected to the opposed end. A liquid supply connector is mounted in a side wall of the casing with a supply port extending therethrough. A nozzle member extends into the casing and includes a gas passage running its full length into a reduced diameter portion of the casing. A receiving chamber is formed between a recessed portion of the nozzle member and an enlarged diameter portion of the casing. The casing further includes an annular constricted middle portion defining a liquid outflow passage around the circumference of the nozzle. The liquid outflow passage provides fluid communication between the receiving chamber and the reduced diameter portion of the casing. Air and fluid are mixed in the reduced diameter portion of the casing.
The apparatus disclosed in the '087 patent has several shortcomings. First, it is difficult to manufacture such spray nozzles so that each has the same output flow characteristic. This is because of the difficulty in manufacturing the spray nozzle with tolerancing so that each includes a liquid outflow passage having the same cross-sectional area. Second, is the less than optimal location in which the mixing of air and liquid occurs, i.e., in the reduced diameter portion of the casing. It has been determined that more efficient and more thorough mixing of air and fluid can be effected if it is caused to occur at a location above the radially restricted portion of the casing.
A third disadvantage is the decay in performance of the spray nozzle over time. This is primarily caused by the buildup of minerals, such as dissolved calcium, which block the relatively small outflow passage. This problem is exacerbated when, as in the disclosed embodiment, the nozzle member includes a forward end that is in contact with the constricted middle portion of the casing and has a number of passage parts in a peripheral wall of the forward end of the nozzle member.
There is clearly a need in the art for a spray nozzle with improved spraying efficiencies. There is also a need in the art for a spray nozzle which can be manufactured so that each spray nozzle produced has consistent spraying characteristics. In addition, there is a need for a spray nozzle that reduces or eliminates the deleterious effects that dissolved minerals will have on the performance of the nozzle over its operational life.